1. Field of the Invention
The present process relates to fatigue damage resistant wire and, in particular, relates to a method of manufacturing a metal or metal alloy wire that demonstrates improved fatigue damage resistance properties, and metal or metal alloy wires made in accordance with such method.
2. Description of the Related Art
It is well known that most of the transition metals tend to be anisotropic, whereby the rheological, tribological, mechanical, and electrical properties vary with crystal direction. It follows that such properties of a metallic material are all dependent on the presence of crystallographic texture (or lack of texture) in the atomic lattice, or crystal arrangement, of the metallic material.
Grain size has been known as an important variable in defining material strength, fracture toughness and fatigue strength for the majority of the twentieth century. For example, gas turbine engines use rotors comprising single metal crystals or large directionally grown crystals to engineer strength in desirable application-specific orientations, such as to create a turbine blade with high strength along the radial direction to prevent or minimize damage during high speed spinning in which high stress is found along the radial direction.
Conventional wire materials have a microstructural cell size (often referred to as crystal size or grain size) that, after processing, is on the order of microns (μm) through millimeters. A micrograph is shown in FIG. 1 of a representative conventional implant grade wire made of 35N LT® alloy (35N LT® is a registered trademark of Fort Wayne Metals Research Products Corporation of Fort Wayne, Ind.), having a grain size on the order of 3 to 12 μm.
Typical medical grade wires are made of biocompatible implant grade materials, including alloys complying with the chemical compositional requirements of ASTM F562. Such wire materials include Co/Cr/Ni/Mo materials including 35 wt % Co-35 wt % Ni-20 wt % Cr-10 wt % Mo, and MP35N® alloy (MP35N® is a registered trademark of SPS Technologies, Inc. of Jenkintown, Pa.). Other biocompatible implant grade materials include nickel-titanium (NiTi), binary shape memory material including Nitinol, as well as Nitinol tertiary alloys (nickel-titanium with additions such as chromium, tantalum, palladium, platinum, iron, cobalt, tungsten, iridium and gold), as well as platinum and alloys of platinum, titanium and alloys of titanium, 300 series stainless steel, and other materials.
Significant research has been dedicated to understanding how alloys such as NiTi behave in the body from the viewpoint of a biological host response, but much less has been published that quantitatively correlates structure with mechanical properties.
These materials may be manufactured by forming a relatively thick piece of hot-worked rod stock from a melt process, and are further processed into wires by drawing the rod stock down to a thin diameter wire.
During each drawing process, often referred to as a “cold working” process, the wire is pulled through a lubricated die to reduce its diameter. The deformation associated with wire drawing increases the internal stress, or stored energy, in the material and tends to decrease ductility. The internal stress eventually must be relieved by various methods of heat treatment or annealing at elevated temperatures to restore ductility, thus enabling the material to be further cold worked to a smaller diameter. Conventional wire annealing typically results in several stages of stress relief, such as: dislocation annihilation, grain nucleation, and grain growth and a generally random crystal orientation distribution. The various material or fiber “textures” that are generated during cold wire drawing are mostly eliminated during conventional annealing and recrystallization. These iterative processes of cold working and annealing may be repeated several times before a wire of a desired diameter is produced and processing is completed.
Although wires made in accordance with foregoing process typically demonstrate excellent fatigue damage resistance properties, further improvements in fatigue damage resistance properties are desired.
Various methods of forming submicron or nano-grained metallic materials are known. In some methods, nanoscale metal powders are formed by hot isostatic pressing (HIPing) to consolidate the powders into a desired shape.
Several additional methods are versions of severe plastic deformation (SPD), in which a metallic material is subjected to plastic strains of more than 600% to 800%. Each of these methods is characterized by the cross-section of the work piece remaining constant before and after the SPD. In high pressure torsion (HPT), a continuous shear is applied to a work piece, such as by placing the work piece between two anvils that are rotated with respect to each other to generate the shear via frictional traction forces. In equal channel angular pressing (ECAP), a special tool having intersecting channels is used to subject a work piece to simple shear while maintaining the cross section of the work piece, and the work piece may be subjected to several ECAP steps to reach a desired degree of plastic deformation. In cyclic channel die compression (CCDC), a work piece is deformed in a special die that corresponds in shape to the work piece, wherein the work piece is first oriented 90° in the die with respect to the nominal shape of the die that corresponds to the shape of the work piece. The work piece is deformed in the die to change to the shape of the die, followed by rotating the work piece 90° and repeating to reach a desired degree of severe plastic deformation.
Although these methods may be suitable in certain applications for materials of certain shapes, these methods are not suitable for the production of metal or high surface quality continuous metal alloy wires of fine diameters such as less than 1.0 mm, for example.
What is needed is a method of manufacturing a metal or metal alloy wire that demonstrates improved fatigue damage resistance properties, and metal or metal alloy wires made in accordance with such method.